The present invention is directed to an arrangement for sealing a heat treatment apparatus comprises a vessel arranged for vacuum and high pressure operation. A container for the charge to be treated is located within the vessel with a heating device located about the container and thermal insulation enclosing the heating device and the container. Separate doors are arranged to close each of the vessel, the thermal insulation and the container.
Such an apparatus is used for sintering materials. The basic principle of a sintering plant for heat treatment of charges subjected to a vacuum with subsequent hot isostatic after treatment is disclosed in DE PS 30 14 619, DE OS 36 21 996 and in U.S. Pat. No. 4,398,702. A container forming the treatment space is located within a vessel arranged for vacuum and high pressure operation. The container is surrounded by a heating device. Insulation is installed between the heating device and the vessel for thermal shielding. Doors or covers are provided at one end of the container and at the corresponding ends of the thermal insulation and the vessel so that the container can be charged and discharged.
When the heat treatment in such an apparatus is completed, the treated charge must be cooled down. Rapid and effective cooling is required for economical and technological reasons, to keep the cycle time as short as possible. Cooling at higher pressure ranges is effected by convection. A more rapid cooling by gas circulation can be gained by opening the container door after treatment for obtaining a high conductance value. During such a procedure, however, good thermal insulation between the container and the vessel should be maintained, since the vessel wall among other reasons must be kept at a lower temperature for strength reasons. Accordingly, the insulation door should be kept closed during the cooling step.
In many sintering operations, the parts to be sintered are extruded or pressed together with binders, such as paraffin or polyethylene glycol or with thermoplastics, for imparting a sufficiently high basic strength to such parts, however, it is necessary to expel these binders prior to the sintering operation. Accordingly, the container door must remain closed, to prevent binder vapors from reaching the heating region and penetrating into the thermal insulation and impairing its effectiveness. A separate vapor line is provided between the container and an external condenser for aspirating the binder vapors.
When operating with compressed gas and when allowing compressed gas to enter, the container door must be kept open. This is necessary for safety reasons, since otherwise the container would be crushed by the pressure. Moreover, maintaining the container open assists in more rapid cooling, as pointed out above.
A closure or sealing device for heat treatment furnaces is set forth in DE GM 87 14 544 and serves for closing the exterior vessel, the thermal insulation and the container. This device has the disadvantage that the vessel door can only be opened along with the insulation door and the container door. As a result, control of the position of the closures or seals can only be effected with great difficulty or not at all. Another disadvantage is that the heat stresses occurring during the heat treatment process can be absorbed only with difficulty.